1. Field of the Invention
The present invention relates to photopolymerizable composite resin compositions for dental restoration having improved physical and mechanical properties, and biocompatibility, and long time sustainability after operation. More particularly, the invention relates to new photopolymerizable composite resin composition for dental restoration i) based on the multifunctional prepolymer mixture of 2,2-bis-(4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)propane (xe2x80x9cBis-GMAxe2x80x9d) and multifunctional prepolymer formed by substituting hydrogen atoms in hydroxyl group with methacrylate groups in the Bis-GMA molecules, and ii) comprising a diluent, an inorganic filler, a photoinitiation system, and other additives.
2. Description of the Prior Art
Polymethylmethacrylate (PMMA) is one example of a dental polymer material that is used as a denture base, an impression material, an adhesive, a dental restoration material and so on, of which the latter is most frequently used.
A mercury amalgam has been generally used as dental restoration materials up to date. It is reported that mercury amalgam can be easily applied, and that it has superior mechanical physical properties such as abrasion resistance and mechanical strength. But it is also a distinctly different color from that of natural teeth, and has poor adhesion with teeth tissues. In addition, it is reported that mercury amalgam is harmful to the human body due to the long term gradual outflow of mercury.
Accordingly, many recent studies have been conducted to develop materials to complement the defects of mercury amalgam or to substitute it for something else. Acrylic resins, which were first used as polymer material resins and have superior mechanical properties in terms of strength, color stability, and water-resistant stability when compared to silicates that have been developed since mercury amalgam are defective in that they have poor abrasion resistance and high shrinkage upon curing. In order to overcome these defects, high filling composite resins using inorganic fillers as reinforcement are developed as dental restoration materials.
A photopolymerizable composition for dental restoration is conventionally composed of an inorganic filler, a prepolymer, a diluent, a photoinitiation system (a photoinitiator and a reductant), and other additives and so on. The composition should meet the requirements of mechanical strength to support the high biting pressure generated when chewing food, the coefficient of heat expansion similar to the tooth, and a polymerization shrinkage low enough to inhibit exfoliation from tooth upon polymerization-curing. Together with its physical properties, the composition should also be the same color and gloss of the natural tooth, and provide a natural tongue-touch feeling of the restored tooth.
2,2-bis-(4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)propane (Bis-GMA), which is a dimethacrylate, is most generally used as a prepolymer of a photopolymerizable composition for dental restoration. The Bis-GMA is principally low in volatility and polymerization shrinkage. A polymer prepared from the Bis-GMA has the advantages of superior strength and, thus, the Bis-GMA is used as a matrix resin. U.S. Pat. No. 4,102,856, U.S. Pat. No. 4,131,729, and U.S. Pat. No. 3,730,947 and so on describes the use of the Bis-GMA. However, the Bis-GMA has high viscosity that requires the addition of a diluent such as triethylene glycol dimethacrylate (TEGDMA) since it is advantageous that a prepolymer such as the Bis-GMA should have low viscosity in order to be efficiently mixed with an inorganic filler. Also, moisture absorption due to hydroxyl group (xe2x80x94OH) in the molecular structure of the Bis-GMA makes the physical property or aesthetics of the cured substances impermanent.
The dental composite material includes a dental composite filling material for dental restoration to fill a cavity caused by dental caries, a crown material, a coalescence material, a dentition correction material, and an artificial tooth material. U.S. Pat. No. 3,066,112 describes various compositions for early dental composite resin, which have not been used in practice since there were various defects on restoring posterior teeth. An amalgam formed from a silver alloy and mercury has been in use from before 1900 as a dental restoring material, but it has been gradually substituted with a material of organic polymer due to its dangerous side effects to humans and circumstances.
The first dental composite resin was prepared by mixing the PMMA powder and a methylmethacrylate (MMA) monomer by Kulzer Corp. (Germany) in 1942 and has been clinically used. The acrylic resin has been used for a long time. However, an organic polymer has such advantages as aesthetics, operation simplicity, and low toxicity, but the polymer itself lacks such physical properties as hardness, strength, and abrasion resistance to support against chewing. Thus, a composite resin compounded with an inorganic filler was developed.
Brown developed a chemical initiating type of commercial composite resin in 1962. Due to the development in the 1970s of ultraviolet-photopolymerization and, subsequently, to the development in the 1980s of visible ray-photopolymerization by ICI (England), polymer composite materials have encroached on conventional amalgams, and their use has increased dramatically.
Dental restoration material is used in anterior filling, posterior filling, cervical erosion filling, fractured porcelain repair, bracket bonding, core building, anterior interdental diastasis treatment, discoloration- and coloration-tooth treatment, and porcelain laminate bonding and so on. In addition to the restoration of dental caries, this dental restoration material is applied in various kinds of dental treatment, such as bonding and coalescense, with an increasing demands for various kinds of aesthetic treatment.
This polymer composite resin has, thus, secured itself a position as a dental restoration material as a result of the above-mentioned uses. However, it still must be improved with respect to the strength, hardness, polymerization shrinkage, water absorption, and solubility, toxicity and aesthetics of the cured substance made of the material.
It is an object of the invention to provide a photopolymerizable composite resin composition for dental restoration having higher photocuring conversion, strength, and hardness as well as physical and mechanical properties such as low polymerization shrinkage and water absorption, and improved biocompatibility than the conventional composite resin composition for dental restoration.
Other objects and advantages of the invention will be clarified in the following detailed description of the invention.
Hereinafter, the present invention is described in detail.
The object of the invention is achieved by means of a photopolymerizable composite resin composition for dental restoration that
i) has improved physical and mechanical properties and biocompatibility;
ii) is based on the multifunctional prepolymer mixture of 2,2-bis-(4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)propane (xe2x80x9cBis-GMAxe2x80x9d) and at least one multifunctional prepolymer containing multimethacrylate groups formed by substituting hydrogen atoms in hydroxyl groups with methacrylate groups in the Bis-GMA molecules; and
iii) comprises an adequate amount of a diluent, an inorganic filler, a photoinitiation system and other additives.
Generally, the Bis-GMA has been most frequently used as a prepolymer for dental restoration because of its superior physical properties, such as high strength after curing. The Bis-GMA molecule has two hydroxyl groups that play a role in promoting an affinity between an organic resin and an inorganic filler, whereas said hydroxyl groups have a property to absorb water due to its high hydrophilicity. In cases where an organic resin absorbs water, the physical properties and aesthetics of a photocured substance is gradually reduced. Thus, if a polymerized resin is swelled by water-absorption, the binding force between it and the filler is weakened so that the filler particle is likely to separate from the resin, thus weakening strength and abrasive resistance. In addition, cytotoxicity is caused, or food absorbs in the restored substance and becomes discolored.
The present inventors have conducted extensive research to improve the aforesaid problems of conventional photopolymerizable restoration material prepared by using only Bis-GMA prepolymer. We have found that a photopolymerizable composite resin composition for dental restoration can be prepared from a prepolymer mixture of Bis-GMA and trifunctional methacrylate prepolymer (Tri-GMA) and/or tetrafunctional methacrylate prepolymer (Tetra-GMA) having reduced hydrophilicity which is formed by substituting at least one hydrogen atoms in the two hydroxyl groups with methacrylate groups in the Bis-GMA molecule. We have also found that the polymerization shrinkage and water absorption that causes physical and mechanical properties and aesthetics deterioration of the resulting photocured substance can be reduced.
The first embodiment of the present invention provides a photopolymerizable composite resin composition for dental restoration comprising: 
(A) 2 to 40 wt % of the prepolymer mixture comprising 2,2-bis-(4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl)propane (xe2x80x9cBis-GMAxe2x80x9d) of formula 1: 
Tri-GMA of formula 2:
(B) 1 to 20 wt % of a diluent;
(C) 40 to 95 wt % of an inorganic filler;
(D) a photoinitiation system; and
(E) other additives,
wherein wt % of all the components are based on the total weight of the composition.
In accordance with the first embodiment, the weight ratio of Bis-GMA of formula 1 to Tri-GMA of formula 2 is 95:5 to 5:95.
The second embodiment of the present invention provides a photopolymerizable composite resin composition for dental restoration comprising: 
(A) 2 to 40 wt % of the prepolymer mixture comprising Bis-GMA of formula 1 and Tetra-GMA of formula 3:
(B) 1 to 20 wt % of a diluent;
(C) 40 to 95 wt % of an inorganic filler;
(D) a photoinitiation system; and
(E) other additives,
wherein wt % of all the components are based on the total weight of the composition.
In accordance with the second embodiment, the weight ratio of Bis-GMA of formula 1 to Tetra-GMA of formula 3 is 95:5 to 5:95.
The third embodiment of the present invention provides a photopolymerizable composite resin composition for dental restoration comprising:
(A) 2 to 40 wt % of the prepolymer mixture of Bis-GMA of formula 1, Tri-GMA of formula 2, and Tetra-GMA of formula 3;
(B) 1 to 20 wt % of a diluent;
(C) 40 to 95 wt % of an inorganic filler;
(D) a photoinitiation system; and
(E) other additives,
wherein wt % of all the components are based on the total weight of the composition, and the prepolymer mixture consists of 90 to 5 wt % of Bis-GMA of formula 1, 90 to 5 wt % of Tri-GMA of formula 2, and 90 to 5 wt % of Tetra-GMA of formula 3.
In accordance with the photopolymerizable composite resin composition for dental restoration of the invention, Tri-GMA of formula 2 and Tetra-GMA of formula 3, constituting the prepolymer mixture, may be synthesized by substituting at least one hydrogen atoms in the two hydroxyl groups with methacrylate group in Bis-GMA molecules of formula 1. Thus, scheme 1 shows that Tri-GMA and Tetra-GMA may be quantitatively synthesized by reacting Bis-GMA with methacryol chloride in the presence of organic amine, for example triethylamine. 
The synthesized multifunctional prepolymer mixture is separated into the individual of Bis-GMA, Tri-GMA, and Tetra-GMA through the column using a developer of mixture of ethyl acetate and n-hexane (50:50 weight ratio).
In accordance with the present invention, the photopolymerizable composite resin composition for dental restoration comprises a prepolymer mixture in an amount of 2 to 40 wt % based on the total weight of the composition.
In accordance with the present invention, the composition comprises a diluent to reduce the viscosity of the prepolymer mixture. The suitable examples of the diluent are methyl methacrylate, ethylene glycol dimethacrylate (EGDMA), diethylene glycol dimethacrylate (DEGDMA), triethylene glycol dimethacrylate (TEGDMA), 1,4-butanediol dimethacrylate, 1,6-hexanediol dimethacrylate, 1-methyl-1,3-propanediol dimethacrylate or 1,6-bis(methacryloloxy-2-ethoxycarbonylamino)-2,2,4-trimethylhexane. The composition comprises a diluent in an amount of 1 to 20 wt % based on the total weight of the composition.
In accordance with the present invention, the composition comprises an inorganic filler in order to improve the mechanical property of the composition and to make the composition opaque to X-ray. The inorganic filler is preferably quartz, barium glass, barium glass/silica, barium glass mixture, quartz/barium glass, silica, zirconia/silica, silica mixture, alumino silicate, lithium alumino silicate and barium aluminosilcate with a particle size of 0.005 to 20 xcexcm, surface-treated with a silane coupling agent, and added in an amount of 40 to 95 wt % based on the total weight of the composition.
In order to surface-treat the inorganic filler, a silane coupling agent is primarily used. The representative examples are xcex3-methacryloxy propyl trimethoxysilane (xcex3-MPS), vinyl triethoxysilane, dimethyl dichlorosilane, hexamethylene disilizane, dimethyl polysiloxane and so on.
The composite resin composition for dental restoration of the present invention is exposed to visible rays that are unharmful to the human body so that a radical is formed from a photoinitiator and a catalyst. Said radical initiates polymerization of a monomer for curing the composition. Polymerization primarily occurs by exposure of the photoinitiator such as xcex1-diketone aliphatic and aromatic carbonyl compound and tert-amine catalyst to the visible ray under a wavelength ranging from 400 to 500 nm. The photoinitiation system consists of a photoinitiator and a reductant. The photoinitiator is preferably camphorquinone (CQ), and added in an amount of 0.1 to 5 wt %. If CQ is photo excited to extract hydrogen in the reductant, the reductant practically initiates radical polymerization. A reductant such as N,N-dimethylaminoethyl methacrylate (DMAEMA) or ethyl p-dimethyl aminobenzoate (EDMAB) is added in an amount of 0.1 to 5 wt % based on the total weight of the composition.
Other additives such as a polymerization inhibitor, a lightstabilizer, an antioxidant, and a pigment to conform the color of the composite resin may be added. A polymerization inhibitor such as hydroquinone (HQ), hydroquinone monomethyl ether or hydroquinone monoethyl ether may be added in an amount of 0.1 to 10 wt % based on the total weight of the composition. A lightstabilizer such as Tinubin may be added in an amount of 0.01 to 5 wt % based on the total weight of the composition. An antioxidant such as Iganox and 2,6-di-tert-butyl-4-methyl phenol butylated hydroxytoluene (BHT) may be added in an amount of 0.01 to 5 wt % based on the total weight of the composition. Inorganic pigments of yellow, navy blue, or red colored-iron oxides and titanium dioxide may be added in an amount of 0.005 to 0.5 wt % based on the total weight of the composition.
The physical properties of the specimen from the resulting composition for dental restoration are estimated as follows:
1) Photoconversion
A photopolymerization efficiency caused by the visible ray is estimated by means of infrared absorption spectroscopy. The conversion of the methacrylate monomer is calculated by measuring the decreased area of the absorption band at 1638 cmxe2x88x921 by the aliphatic double bond on the basis of the area of the absorption band at 1609 cmxe2x88x921 by the aromatic ring.
2) Polymerization Depth
A 4 mm (diameter)xc3x9710 mm (height) metal mold is placed on a transparent film covered with a white paper filter and subsequently filled with a restoring material free of air bubbles. A transparent film is then placed on the top of the mold, and excessive material is removed by pressurization. By photo irradiation, the polymerization depth in accordance with the exposure time is measured to an accuracy of 0.01 mm with a micrometer.
3) Polymerization Shrinkage
A cylinder shaped specimen (6.0xc3x973.3 mm) is placed in a transparent glass mold, and cured with a light-irradiator. Density of the specimen is measured with a picnometer before and after curing the specimen, and calculated according to the following formula.       Polymerization    ⁢          xe2x80x83        ⁢    shrinkage    ⁢          xe2x80x83        ⁢          (      %      )        =            (              1        -                              ⅆ            uncured                                ⅆ            cured                              )        xc3x97    100  
4) Water Absorption and Solubility
A composite resin composition is made into about a 6 cm (diameter)xc3x973 mm (height) specimen, which is cured. The weight of the cured specimen is measured, and then the cured specimen is dipped into distilled water at 37xc2x0 C. After every 24 or 48 hours, the specimen is then taken out, water is removed from the specimen, and the weight of the specimen is measured. Moisture absorption is calculated by the following formula.       Water    ⁢          xe2x80x83        ⁢    absorption    ⁢          xe2x80x83        ⁢          (      %      )        =            (                                    weight            ⁢                          xe2x80x83                        ⁢            after            ⁢                          xe2x80x83                        ⁢            dipping                    -                      weight            ⁢                          xe2x80x83                        ⁢            after            ⁢                          xe2x80x83                        ⁢            cure            ⁢                          xe2x80x83                        ⁢            before            ⁢                          xe2x80x83                        ⁢            dipping                                    weight          ⁢                      xe2x80x83                    ⁢          after          ⁢                      xe2x80x83                    ⁢          cure          ⁢                      xe2x80x83                    ⁢          before          ⁢                      xe2x80x83                    ⁢          dipping                    )        xc3x97    100  
In order to measure solubility, the specimen is taken out, and the water is removed from the specimen. The specimen is completely dried again in a desiccator to have uniform weight, and the weight of the specimen is measured. Solubility is calculated by the following formula.       Solubility    ⁢          xe2x80x83        ⁢          (      %      )        =            (                                                                                    weight                  ⁢                                      xe2x80x83                                    ⁢                  after                  ⁢                                      xe2x80x83                                    ⁢                  cure                  ⁢                                      xe2x80x83                                    ⁢                  before                  ⁢                                      xe2x80x83                                    ⁢                  dipping                                -                                                                                        weight                ⁢                                  xe2x80x83                                ⁢                after                ⁢                                  xe2x80x83                                ⁢                dipping                ⁢                                  xe2x80x83                                ⁢                and                ⁢                                  xe2x80x83                                ⁢                complete                ⁢                                  xe2x80x83                                ⁢                drying                                                              weight          ⁢                      xe2x80x83                    ⁢          after          ⁢                      xe2x80x83                    ⁢          cure          ⁢                      xe2x80x83                    ⁢          before          ⁢                      xe2x80x83                    ⁢          dipping                    )        xc3x97    100  
5) Radio-opacity
Specimen (13 mm [diameter]xc3x972 mm [thickness]) is prepared, and is placed together with an aluminium step-panel (purity: 99.9%, thickness: 2 mm) on X-ray film and radiated for 0.5 sec in 65xc2x15 kvp and 15 mA, developed, and measured with a densitometer, and compared with the step-panel.
6) Diametral Tensile Strength
The diametral tensile strength measurement method, in which stable compression stress is applied to a specimen instead of direct tensile strength, is used especially in the measurement of the physical properties of dental material. In this method, a disk shaped specimen is laid horizontally, and compression weight is applied to the specimen to cause tensile stress interior of the specimen. A 6 mm (diameter)xc3x973.6 mm (thickness) specimen is prepared, and stress is applied to the specimen in cross-head speed of 0.5xc2x10.2 mm/sec with a tensile tester until the specimen is fractured. The diametral tensile strength is calculated by the following formula.       Diametral    ⁢          xe2x80x83        ⁢    tensile    ⁢          xe2x80x83        ⁢    strength    ⁢          xe2x80x83        ⁢          (      DTS      )        =            2      xc3x97      maximum      ⁢              xe2x80x83            ⁢      load              π      xc3x97      diameter      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      specimen      xc3x97      thickness      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      specimen      
7) Flexural Strength
Photo irradiation is conducted on both sides of a mold (25 mmxc3x972 mmxc3x972 mm) to produce a specimen, which is kept in distilled water at 37xc2x0 C. for 24 hours. Stress is applied to the specimen in cross-head speed of 0.75xc2x10.25 mm/sec with tensile tester until the specimen is fractured. The flexural strength is calculated by the following formula.   σ  =            3      xc3x97      maximum      ⁢              xe2x80x83            ⁢      load      ⁢              xe2x80x83            xc3x97              xe2x80x83            ⁢      distance      ⁢              xe2x80x83            ⁢      between      ⁢              xe2x80x83            ⁢      supporters              2      xc3x97      area      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      specimen      ⁢              xe2x80x83            xc3x97              xe2x80x83            ⁢      height      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      specimen      
8) Cytotoxicity
Cytotoxicity of a composite resin is estimated by comparing the toxicity degree according to the agar layered plate method. 10 mm (diameter)xc3x972 mm (thickness) specimen is tested using polyvinylchloride [PVC, response rate: 3/4] as the positive control group and polyethylene (PE) as the negative control group. The specimen is first adhered to an L-929 cell suspension using Eagle""s agar medium, and incubated for 24 hours at a temperature of 37xc2x0 C. in a 5% CO2 incubator. The cell lysis ratio is measured in a discolored region of the specimen and is indicated as a zone index and lysis index, as listed in Table 1, from which a response index (RI=zone index/lysis index) is calculated. Cytotoxicity is evaluated from RI as it is listed in Table 2. The lower the value, the lower the toxicity.